Screen grid circuit



'Jam 1, 1935- w VAN B. ROBERTS 1,985,952

,SCREEN GRID CIRCUITS Filed Feb. 5, 1950 S M T R E o R. RR.. O TN NAVH WR N,E lu 7 A W, Vl B ATTORNEY Patented Jan. 1, 1935 l l `1,935,952 scnnm-cnmcmcmr Walter van Bf. Roberts',;rrinceton,

N. J., ssignai- ,i

to Radio Corporation of America, a corporation of Delaware My present!inventionv -re'latesf to ampliiiercircuits, and'more particularlyv to tunedxlradio fre-1V quency ampliiiercircuitsf employingv electron discharge devices of the yscreen gridtype.1 51 As-fisswell known/to `thoseI skilled1inv the'art,

the-j "screen grid* tube1v Was devised to' reduce', to' a' considerable extent, thev undesired coupling effect between' ythe control electrode andfanodeiofi theusualf triode. Such a screen grid device,v or' 101 tetrode, usually has its screen element connected:

to` an invariable source. ofc positiverpotentiali-v tov' prevent'th'e electric eld' of the anodeA from` aiecting the control electrode:

' Wlien a screengridtubeis employed infastage 15'? otituned radio frequencyl amplification; itis'cus-V 'cuitsrfromeach othersozfthatzcoupling, duefto stray.l capacities, J is;:prevented.. However it: has' been found that-'am imperfectionv in .thel shielding? siredr' coup1ing. tozfaifect:the?arnplifyingrpouert` of the stage." extremely ismall residual capacity? of the orderofonemicrocinicro-faradiforiless; easily v produced' bylawslight imperfection'V in the 25@ shielding! device; is'ffoundbtoreduce th'esta'bilityYY oi'foperation?-ofthefscreen."grid;:stage:1Y i Now, Ih'ave' devised aimetlodzotandaarranged' meanslifors. achievingr thef ylast' step required tof` make i the f stability' otiscreen gridstagesjperfectc 3,03 Tlfatisto fsay, by :my'present'inventionnit` '-iszpos v sible stof completely.""el-iminate'thesbad effects* of interstage'i Pcapacity" coupling; Wherebyi the. 'opi-i erati'glequivalent L ot'oner hundred:`v per' cent' j shielding:isfsecured;l "Accordinglyr one'fofarthemain objectsi of? myf presentlinventioniisltoprovide a 4method for; and'` f mean'szfon, 'cdmpletely.'"'stabili'zingr ai tunedtradio frequency amplifier employing ai screen; grid tubeg:whichlconsists` iniemployingv in V4therinput 40) and/on outputcircuitsf off the ampliiier` aumeans fonfoisetting the,tendencyf'tofoscillate arising by' virtue ofzithe 2 interstate 1 capacit'ycoupling-frei#- mainin'g after-fthe inputzand' output circuits' have beeneirnperfectly shielded fr'oinzeachizother; f Another-f important object: jofaglthe inventiom is g toprovide aLrnethod:` o'fgfandmeans for, perfectlylv stabllizinga tunediradio frequency amplier'utilizing a tube of the screen gridtype,-which2methV` odslgfconsist lin' employing', in: the `input'.Y and/ orA output."circuitslvof.` thek tetrode, .arrangements for; neutralizing: the1 residualiv intersta'ge capacityV coupling; remaining` after the inputf and: output circuit'sfhavefbeenfmcre"or:lessiperfectlygshielded fnomtoneganother;which arrangements-.'comprise` 55 meansifforfffeeding `,baokf energyfto the:l input- -cir..

cuit degenerative phase to *completely eliminatefoscillations produced by the aforementioned? residual capacity coupling. 1 f g Still"1 other objects of the invention are to ploying-` Vscreen grid tubes,v and particularly tuned radio y frequency'amplifers @employing l imperfectv shieldingsfor' the inputand outputfcircuits. and? to providearrangements in such anfamplifler Iori 'overcomingthe e'ects *of iresidual-interstagef ca.; 10 pacity coupling-'produced byv the? imperfections in shielding, an'dwhiclfiy arrangernents'arejnot f only reliable in operation but easily and econ'o'ln` icallyf appliedifto the amp1ier.

' The novelfeatres which IY believe to be charac#v 15:3 teristic of my l invention' are set? forth in particu; larity inthe appendedclaims, tHe inventionitself,l however;- as to both' itsforganzation 'and'methodof' operation Willbest Vbe understoodv byiferenc 5to' the following descriptiontaken'in connection with 20 the *drawing i'n' which Ihav'eindicateddiagram; maticallfy'iseveral circuit' organizations whereby myinvention.maybecarriedfintoeffect- Irrthe'drawin'g,`f

lig.. 1` shows 'a stage of tuned radio frequency 255 amplication employing a screen gridltl'ibe';v andi` embodyinga'form oithevinventionf. f

v Fig.12.isa circuitsimilar-torig-`1;andfembodyingf'a modied arrangement, l j f ingrthezpreferredformof thein'vention.

. Referring: now to-th'e'accompanyingdrawing which; like characters Lof :reference: indicate' the" same Aparts in the different iiguresgl'ig. f l shows a stage oftuned'f radio frequency' amplification em- 35@ ployingsaxscreen grid tube. The incoming signal-k energyiskapplied through'a transformercoupling 1g to theatuned input circuit;2L of :thetube 3:.21 .The cathodeof thetube is showncfor the sakeofsim` plicity, heated: :by a direct current; source-FA; it 40S wbeingunderstoodthat a source ofalternatingcurt 1 rent could also be used for the same purpose.: The

anodef of. the itube. is -connectedto the positiverternivinalfof asource4 B.' 4through the primary 4of the, soutputjtransformer, thesecondaryfoflthe said 45" circuito; the "suceeedingsta'ge.` i A Y n'Ihelatter isconnected toany Well-'knowntypel of utilizing means; such as afdetector, subsequent 'transformer -being vconnected infth'e tuned input l .audiolfrequencyamplifiers and' loud-speaker,l thev 50T` la'tte'rf:notibeing shown` in 'order' to preserve sim';

only to thefsubject matterof therinvention 1 "I'h'e4 screeniygridrelement 5; is disposedbetween the anode: and controlzelectrode;fand;is also disposed 555 plicity :ofi` disclosure' and` confiney the descriptionv likewise confined Within a similar shielding device' 8. However, due to various imperfections inthe influence of the shielding devices, and also due to coupling between the lead wires to the electrodes within the tube 3, a residual interstage capacity coupling 9 is found to exist between the input and output circuits of the stage.l `The shielding device and the interstage capacity coupling are. shown in dotted lines in Fig. 1.

One effective method of eliminating they effect of the residual interstage coupling 9, which coupling tends to give rise to oscillations thus impairing the efficiency of thestage, `is to insert a high resistance `R. in the control electrode connection.

ofthe tube. The` high frequency oscillations re- 'ferred to flow through such a'resistance, ,and the amount of couplingoffered by the capacity 9 is not suicient to sustain oscillations through such a high resistance. Another effective method ofI overcoming `the effectv of the capacity 9 is to in-V crease the resistance of the primary coil 4 in the Aanode circuit of the tube, as'by making the primary of very finewire of some high resistance material.r Sinceithis coil 4 is practically never part ofthe tuned circuit, its resistance may be increased many times without interfering with normal tuning.` Such arrangements may be classed under the general term of losser arrangements, and :it

may be employed to overcome the effect ofthe residual interstage capacity coupling.

In Fig. 2 I have shown another arrangement for overcomingthe effect of theresidual interstage capacity coupling 9. This arrangement consists in feeding back energy from the grid circuit of the tube 3 in degenerative phase to the output circuit. This is accomplished by inserting an additional coil 10 in the input circuit of the tube, and coupling `this coil to the secondary ofthe input transformer 1. ,Condenser C is connected between the end of the coil 10 and the anode of the tube 3. If the coil loris properly coupled to the input secondary, and

C is of suitable capacity, then the combination C-10 will produce on the anode a voltage just,

equal and opposite to that produced by the combination of the inputlsecondary and the capacity 9.` `ATherefore, the two effects willjust neutralize one anotherand no oscillations will occur.

` vOf course, exactly the same principle Acan*l be used in the reverse manner,` that is by'closfely coupling the coil 10'to the output primary 4, and

connecting thev capacity C. between the 'end of the coil 10 and the grid circuit, thus feeding back energy in degenerative-phase to the tuned input circuit of the tube. Still other combinations can be used, all operating on the same principle, viz,I

the residual interstage capacity coupling Abetween the input and output circuit of the tubeis offset, or neutralized byl another coupling effect of equalr magnitude and opposite phase. It will be found that such degenerative feed back arrangements overcome the oscillations produced by the resid-- will` be apparent yto those skilled in the art that other similar arrangements' van output circuit including a direct current ual capacity coupling 9 in such a manner that practically perfect shielding is secured.

A preferred arrangement that can be used to bring the shielding to perfection is shown in Fig. 3, wherein the stabilizing device is disposed drectly in the screen grid circuit. In this arrangement a condenser C' of some few hundreds of micromicro-farads is connected in -series -in the lead 6 to the screengrid 5, this condenser being shunted by a choke coil L to carry direct current. The capacity reactance in the screen circuit multiplied by the radio frequency current passing through itgives a voltage on the screen which feeds back energy through the screen-to-control- 'grid capacity 11shown in dotted lines in Fig. 3, 'in degenerative phase to the input tuned circuit, thus stabilizingthe amplifier. In other words, it will be noted that in this arrangement the screen grid element possesses a varying potential created by the incoming signal energy itself, this beingafeature distinct fromthatknown in the prior art' wherein it is -necessary for the screen elementv to possess an invariable-positive potential at all times.

All-ofv the arrangements dheretofore described have not been completely satisfactory when used in circuits Where'there was capacity coupling between the input and output circuits of the order often micromicro-farads but any of these arrangements can be easily and effectively applied, with satisfactory results, to the elimination of the effects of an extremely small residual interstage capacity coupling of the order of one micromicro-farad or less, such as results from imperfect shielding in connection with screen grid tubes. It wi1l,.thus,.be apparent that it would be a desirable practice to use shielding between the `inlput and output circuits of each amplifier stage to the extent that 'it is cheap and easy (as by stamping out stock metallic cans or by employingA metallic cylinders having ends open) and to complete the stability `of the amplifier by using arrangements as heretofore described. f

It willbe `seen that by means of the arrangements .before mentioned a method has been devised for overcoming the effects of residual interstage capacity coupling produced in an imperfectly shielded v.amplifier employing screen grid tubes. While I have indicated and described several systems for carrying my invention into effect, ity will be apparent to one skilled in the art that my invention is by no means be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a tetrode, including a screeny grid element and a control electrode, a tuned input l.circuit associated'with said control electrode,

voltage source to which is connected the screen grid element, means for imperfectly shielding the input and. output circuits from each other, and additional means eliminating the effect of the residual interstage capacity coupling of the imperfectly shielded circuits.

l2. In combination, a tetrode including a screen grid element and a control electrode, a tuned input circuit associated with said control electrode, an output circuit including a direct current voltage source to which are connected the anode and screen grid element of said tetrode, means for imperfectly shielding the input and output circuits limited to the particular organizations shown and described, but.

from each other, and additional means including a capacitative path between the output and the input circuits for feeding energy in degenerative phase to the energy owing' through the residual interstage capacity coupling of the imperfectly shielded circuits.

3. In combination, a tetrode including a screen grid element and a control electrode, a tuned input circuit associated with said control electrode, an output circuit, said screen grid element being connected to a fixed direct current voltage point in said output circuit, means for imperfectly shielding the input and output circuits from each other, and additional means associated with the screen grid element and the output circuit for producing a voltage on the screen which feeds back energy through the screen to control grid capacity in degenerative phase to the energy iiowing through the residual interstage capacity coupling between the imperfectly shielded circuits.

4. In combination, a tetrode including ascreen grid element and a control electrode, a tuned input circuit associated with said control electrode, an output circuit including a direct current voltage source to which is connected the screen'grid element, means for imperfectly shielding the input and output circuits from each other, and additional means, including a capacitive reactance in the screen grid circuit, for utilizing the radio frequency current for producing a voltage on the screen which feeds back energy through ythe screen to control grid capacity in degenerative phase to the input tuned circuit.

5. In combination in a tuned radio frequency amplifier stage employing an electron discharge device including a screen grid element, a tuned input circuit associated with thel control electrode and cathode of saidy tube andan output circuit associated with the screen grid and anode of said device, means for imperfectly shielding the input and output circuits from each other, said output circuit including means for applying a constant positive bias to the screen capacitative means, in the screen gridcircuit, for utilizing radio' frequency energy to be amplified for producing a varying voltage on the screen, and a choke coil in shunt with the capacitative means.V n 1 6. In combination, with a source of incoming signals, an electron discharge tube including at least a cathode, a signal grid, an anode and a.

grid when energy to be amplied is not flowing through the amplifier,

screen grid electrode surrounding said anode, an k input circuit tuned to an incoming signal con` nected between the signal grid and cathode of said tube, said tuned input circuit being coupled to said signal source, an output network connected between the cathode and said anode of the tube, said output network including a direct current voltage source, means for connecting said screen grid electrode to a point on said voltage source, means for imperfectly shielding the tuned input circuit and said output network from each other, and additional means, including a resistor in the signal grid circuit of said tube, for eliminating the effect of the residual interstage capacity coupling of the imperfectly shielded circuits.

, 7. An amplier comprising a space discharge tube provided with a cathode, a control grid, an anode and a screen grid, an output circuit as- -sociated with the cathode and anode, an input circuit associated with the control grid and cathode, a source of positive potential for said screen grid, a condenser of low impedance to radio frequency currents in series between said source and the screen grid, and a path having a high impedance to such, currents between said source and screengrid, said second path comprising an inductance coil in shunt with the condenser.

WALTER VAN B. ROBERTS. 

